Traction control systems are well known in the art of motor vehicles for enhancing wheel grip. Such systems are based on the appreciation that in low-traction situations, for example on ice, undesirable or excessive wheel spin may occur. Traction control systems may be of particular importance where an open differential is employed.
Traction control systems initiate traction control interventions at one or more vehicle wheels in dependence on wheelslip. Wheelslip is generally monitored continuously by comparing signals from wheel speed sensors with a vehicle reference speed. Traction control systems can apply brake pressure to a slipping wheel, for example with the aim of transferring torque to other wheels and/or increasing torque applied to other wheels. Additionally or alternatively, if a wheel is slipping, the traction control system may reduce engine torque to help the wheel regain traction.
A traction control system may form part of, or co-operate with, an electronic stability control (ESC) system for maintaining steering control.
While conventional traction control interventions are helpful in most situations, there are certain reduced wheel load scenarios in which they are not.
In one scenario, the load on a vehicle wheel can be reduced when travelling over speed bumps, potholes, or general road undulations at high speed. Indeed one or more wheels of a motor vehicle may become airborne as a result of such terrain. In this scenario, conventional traction control interventions may take unnecessary measures and cause annoyance to the driver, and this may, in extreme cases, lead to a reduction in stability. In particular, if a wheel spins faster as a result of it being in the air and the vehicle is travelling at a high speed, braking the wheel with a traction control intervention can cause it to lock or drop below the vehicle reference speed. When the wheel then lands it will skid undesirably while it is accelerated to the vehicle speed once more. Furthermore, particularly if wheelslip due to the wheel being airborne is significant, a traction control engine torque reduction may be triggered. This may cause an annoyance to the driver and may impede vehicle progress.
A different reduced wheel load scenario may occur when a vehicle is travelling across undulating ground in an off-road mode. In such a scenario a traction control intervention to apply brake pressure to a slipping wheel may be undesirably slow, being a reactive intervention that occurs only after torque has already been lost to wheelslip.
It is an object of the invention to provide an improved traction control system which solves or mitigates at least one problem associated with the prior art.